1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method that form an image on a continuous medium using the electro-photographic process, and more particularly, to an image forming apparatus and image forming method that control the power with which the medium is conveyed by an image forming drum.
2. Description of the Related Art
Image forming apparatuses that employ the electro-photographic process are widely used in printers, copying machines and facsimile machines, etc. These apparatuses form a toner image on an image forming drum such as a photosensitive drum, and transfer the toner image to a printing medium, whereby an image is formed on the printing medium.
In a high-speed printer, in order to continuously carry out the transfer operation onto the medium, it is common to use a continuous medium such as continuous paper for the printing medium. FIG. 22 is an explanatory drawing of a conventional image forming apparatus using such a continuous medium.
Continuous paper having sprocket holes will be explained as an example of the continuous medium. The continuous paper 120 has sprocket holes at both edges, and is conveyed by tractor pins mounted on a tractor 110 located upstream from a photosensitive drum 100. Furthermore, a tractor 112 is located downstream from the photosensitive drum 100 where necessary, such that two tractors 110 and 112 conveys the continuous paper 120.
Because the continuous paper 120 is conveyed synchronously with the tractor pin conveyance speed, the conveyance speed v of the continuous paper is identical to the tractor pin speed. The photosensitive drum 100 is set to rotate at a circumferential speed (v+xcex1) that is slightly higher than the paper speed v, such that the paper upstream from the photosensitive drum is prevented from slackening while variations in the drum diameter and other factors are taken into account.
The continuous paper 120 adheres to the photosensitive drum 100 at the transfer point, and transfer current is applied from the back side of the paper by a transfer charger 102, whereby the toner image on the photosensitive drum 100 is transferred to the continuous paper 120. When the continuous paper 120 comes into contact with the photosensitive drum 100 at the transfer point, the electrostatic attraction is generated between the continuous paper 120 and the photosensitive drum 100 due to the application of the transfer current.
As described above, because the speed of the photosensitive drum 100 is higher than the paper conveyance speed, an appropriate tension force F is applied to the continuous paper 120 upstream from the photosensitive drum 100, enabling preferable paper conveyance. The tractor 110 located upstream from the photosensitive drum 100 generates a reaction force that opposes this tension force F brought about by the photosensitive drum 100.
However, because the paper tension force F is the result of electrostatic attraction between the photosensitive drum 100 and the continuous paper 120, it is not fixed, and fluctuates. When the paper tension force is large, the paper elastically deforms and stretches, changing the transfer conditions. When the paper tension force is excessive, the sprocket holes in the continuous paper may tear, and the paper sections may become separated at the perforations in the paper.
In other words, because the paper tension force is applied at the transfer point by the photosensitive drum 100, if the paper tension force is large, the toner is transferred while the paper is stretched, and the tension is released and the paper returns to its original state after the transfer is performed. If the toner image is transferred while the paper is stretched, it is easy for the toner image to become blurred.
As a result, when the degree of paper tension force fluctuates in regard to a single page, the problem occurs that the toner image varies depending on the paper tension. This variation is not conspicuous in a low-resolution image having a resolution of around 240 dpi, in which the dot diameter is large, but in a high-resolution image having a resolution of around 1,200 dpi, in which the dot diameter is small, the variation becomes noticeable.
Furthermore, when the paper tension force fluctuates considerably, the paper begins fluttering near the transfer point, as shown in FIG. 23, causing the transfer conditions to change more significantly, and the paper also separates from the tractor or tears occur in the continuous paper.
One of the factors involved in this fluctuation is the amount of toner between the photosensitive drum 100 and the continuous paper 120, but the degree of fluctuation in the toner amount has normally been small in the conventional art, and fluctuations in the tension force generally did not become unduly large. However, in recent years, it has become desirable to use continuous paper output for not only internal documents and preliminary documents, but also for such official documents as invoices, receipts and other documents submitted to customers. In order to carry out clearer printing for such purposes, it is required to reduce extra toner fog on the photosensitive drum.
Consequently, due to the increased fluctuations in the amount of toner on the photosensitive drum and in the amount of paper tension, where there is a low printing ratio or little extra toner fog, there is an increased risk of excessive paper tension, leading to blurred characters, separation of the paper from the tractor or tearing of the paper.
Therefore, an object of the present invention is to provide an image forming apparatus and image forming method that maintain at an appropriate level the tension on the continuous medium caused by the image forming drum.
Another object of the present invention is to provide an image forming apparatus and image forming method that prevent the application of excessive tension to the continuous medium while at the same time preventing slack in the continuous medium.
Still another object of the present invention is to provide an image forming apparatus and image forming method that stabilize the characteristics of the conveyance of the continuous medium by the image forming drum.
Still another object of the present invention is to provide an image forming apparatus and image forming method that enable clear printing while maintaining at an appropriate level the tension on the continuous medium caused by the image forming drum.
In order to achieve these objects, the image forming apparatus of the present invention has feeding unit for feeding a continuous medium, toner image forming unit for forming a toner image on a rotating image forming drum, transfer unit for transferring to the continuous medium the toner image on the image forming drum by causing an electrostatic force to act on the image forming drum and the continuous medium at a transfer position via a transfer current, and control unit for monitoring the tension force applied by the image forming drum on the continuous medium and controls the value of the transfer current from the transfer unit in accordance with the results of such monitoring.
The image forming method according to the present invention has a step of forming a toner image on a rotating image forming drum, a step of transferring to a continuous medium the toner image on the image forming drum by causing an electrostatic force to act on the image forming drum and the continuous medium, which is being conveyed, at a transfer position via a transfer current, and a step of monitoring the tension force applied by the image forming drum on the continuous medium and controlling the value of the transfer current from the transfer unit in accordance with the results of such monitoring.
According to the present invention, because the tension force applied to the continuous medium by the image forming drum due to the electrostatic force impressed by the transfer apparatus in order to transfer the toner image from the image forming drum to the continuous medium is controlled, the amount of tension can be maintained at a proper level, the conveyance of the continuous medium can be conducted in a stable fashion, and good printing results can be obtained. Furthermore, because the tension force is monitored and controlled, it can be controlled within an appropriate range.
Furthermore, it is preferred that the control unit of the present invention monitors the printing ratio from the printing data used to form the toner image on the image forming drum, and that it control the value of the transfer current output from the transfer unit in accordance with the results of this monitoring. Because the tension force is proportional to the toner amount, the tension force applied by the image forming drum can be easily measured by monitoring the printing ratio that determines the toner amount.
Moreover, it is preferred that the control unit of the present invention monitors the current supplied to the motor that rotates the image forming drum, and that it control the value of the transfer current output from the transfer unit in accordance with the results of this monitoring. Because the tension force is proportional to the image forming drum motor current value, the tension force applied by the image forming drum can be easily measured by monitoring the motor current value.
In addition, it is preferred that the control unit of the present invention reduce the transfer current value where it is determined from the results of monitoring that the amount of tension is high, and increase the transfer current value where it is determined that the amount of tension is low.
It is furthermore preferred that the control unit of the present invention reduce the transfer current value where the printing ratio is low, and increase the transfer current value where the printing ratio is high.
It is also preferred that the control unit of the present invention reduces the transfer current value where the motor current value is large, and increase the transfer current value where the motor current value is small.
It is moreover preferred that the control unit of the present invention controls the transfer current value in accordance with the results of monitoring of the printing ratio after a prescribed delay interval following the point in time at which such monitoring is performed.
The other feature of the present invention comprises an image forming apparatus that forms a toner image on a continuous medium, wherein such image forming apparatus has feeding unit for conveying the continuous medium, toner image forming unit that forms a toner image on a rotating image forming drum based on image data, transfer unit that transfers to the continuous medium the toner image on the image forming drum by causing an electrostatic force to act on the image forming drum and the continuous medium at a transfer position via a transfer current, and control unit that creates the image data by adding to the picture data to be printed additional data in which black dots are dispersed in order to control the tension force applied by the image forming drum on the continuous medium.
The other feature of the image forming method according to the present invention comprises a step of creating image data by adding to the picture data to be printed additional data in which black dots are dispersed in order to control the tension force applied by the image forming drum on the continuous medium, a step of forming a toner image on a rotating image forming drum based on the image data, and a step of transferring to the continuous medium the toner image on the image forming drum by causing an electrostatic force to act on the image forming drum and the continuous medium, which is being conveyed, at a transfer position via a transfer current.
In this feature of the present invention, because an additional pattern to control the tension force is forcibly added regardless of the pattern to be printed, the generation of an excessive tension force can be prevented using a minimal amount of toner in the additional pattern. Thereby, the continuous medium can be conveyed in a stable fashion, and degradation of the print quality can be easily prevented.